There have been efforts to produce field emission displays (FED), which provide a flat panel display using large-area field electron sources to provide electrons that strike colored phosphor to produce a color image. FED's combine the advantages of CRTs, such as providing high contrast levels and very fast response times, while providing the advantages of flat panel technologies. They also offer the possibility of requiring less power, about half that of an LCD system for example. An FED display operates similar to a conventional cathode ray tube (CRT) with an electron gun that uses high voltage to accelerate electrons which in turn excite the phosphors, but instead of a single electron gun, a FED display contains a grid of individual nanoscopic electron guns. In the past, an FED screen was constructed by laying down a series of metal stripes onto a glass plate to form a series of cathode lines. A series of rows of switching gates is formed at right angles to the cathode lines, forming an addressable grid. At the intersection of each row and column a small patch of emitters are deposited. The metal grid is laid on top of the switching gates to complete the gun structure. A high voltage-gradient field is created between the emitters and a metal mesh suspended above them, pulling electrons off the tips of the emitters. This is a highly non-linear process and small changes in voltage will quickly cause the number of emitted electrons to saturate. The grid can be individually addressed but only the emitters located at the crossing points of the powered cathode and gate lines will have enough power to produce a visible spot, and any power leaks to surrounding elements will not be visible. The grid voltage sends the electrons flowing into the open area between the emitters at the back and the screen at the front of the display, where a second accelerating voltage additionally accelerates them towards the screen, giving them enough energy to light the phosphors. Since the electrons from any single emitter are fired toward a single sub-pixel, scanning electromagnets are not needed.
Although shown to be a viable display technology, past efforts have not produced displays which would allow use in commercial products. In FED devices, strong electric field and high temperature can cause electron emission from a material. In contrast to conduction current, emission current may be low, but the energy of electrons is much higher in emission than in conduction, thus making them useful for a number of applications, like displays or electron microscopy. Emission from flat metal electrodes require very high voltages at room temperatures. On the other hand, sharp needlelike cathodes require lower voltages due to enhancement of electric fields at the tip of an electrode. An example of a sharp material for electron emission are carbon nanotubes. Carbon nanotubes have unique electrical and mechanical properties. Emission from a single carbon nanotube starts at a much lower voltage than a corresponding metal wire of similar dimensions. It has been suggested that the carbon nanotubes have atomically sharp wires dangling from its ends or tips. As compared to a single carbon nanotube, an array of carbon nanotubes' threshold voltage is much higher and its emission current decreased by a large amount.
There have been efforts to use carbon nanotubes (CNT) in such displays or other FED applications. For example, companies like Motorola, Samsung and Cendescent have shown small VGA FED type prototypes in various technical meetings (e.g. Motorola's “Nano-emissive display, 5” diagonal and 3.3 mm thick). However, there are many challenges to achieve uniform field emission from a large area of aligned CNT. In prior efforts, the synthesis of large area aligned CNT with uniform height was not achievable. In such efforts, longer CNT are closer to the anode than the smaller CNT. Therefore emission current from different sections of CNT cathode may be different. Additionally, stray carbon nanotubes may get pulled out of the array forming a resistive contact with the anode, which causes short-circuiting. An additional limitation relates to a screening effect. It has been suggested that the threshold voltage increased due to a screening effect. The screening effect can be thought of as a reduction in an effective electric field at a tip of a needle when other needles with similar potentials are placed within its proximity. Current density achieved from macroscopic samples of carbon nanotubes are of the order of 1 mA/cm2. The emission current from single carbon nanotube of 10 nm diameter was 1 mA. This means that only one thousand carbon nanotubes are effectively emitting from an area of 1 cm2, as compared to 108 carbon nanotubes present. Therefore, a need exists for a field emission display that is more efficient. Due to proximity of neighboring CNTs, electron emission from an array of CNT occurs at much higher voltages as compared to single CNT. This is disadvantageous because higher voltages are then required for desired emission from a CNT array to produce the pixel or sub-pixel brightness or other characteristics as desired. Efforts to overcome these challenges have resulted in different techniques being used, such as like dispersing CNT with an organic binder or screen printing of the CNT array. These methods can create uniform coatings of CNT. However, using these techniques, aligned CNT cathodes cannot be generated. Therefore, a need exists for a field emission display that is more efficient.
Further limitations and disadvantages of conventional, traditional, and proposed approaches will become apparent to one of skill in the art, through comparison of such systems and methods with the present invention as set forth in the remainder of the present application with reference to the drawings.